Dream 2700 | A Tailless Tale
Part I: Where This Came From
This is the story of my own-design, tailless glider — please sit down and relax, is going to be a long story!
Since I was a child, flying has been my dream. Airplanes are my passion since I can remember. I started with control line models, moved to RC models, and after I started flying for real with hang gliders, paragliders, ultralights and sailplanes. Today I own a beautiful LS6 sailplane, and a paraglider. I’ve always dreamed about designing my own tailless glider. Over the years, I’ve been exploring different tailless concepts. This series of articles is dedicated to the evolution of my design concept, from the original idea up to the 1:5 scale model. It’s name is Dream 2700.
Why Tailless?
Because tailless sailplanes look beautiful and simple. Unfortunately, reality is a bit different: they are for sure beautiful, but they are as well extremely complex. They have some very specific advantages, but at the same time there are several issues that must be mitigated with very careful and complex design work.
In anything at all, perfection is finally attained not when there is no longer anything to add, but when there is no longer anything to take away — Antoine de Saint-Exupéry
Advantages
- Their reduced directional stability allows to get in exchange very good spiral stability, and this is a plus for thermal soaring.
- A well-designed tailless sailplane can achieve good stall performance and can be very resistant to spins.
- Friction drag can be minimised.
- The design architecture is ideal for a motorized version. An engine can be easily installed in a pusher configuration (getting as well a stabilizing effect).
- It can be cheap to build, not having a long fuselage and tails. Not so sure about that, though.
- A flying wing is absolutely a fascinating and very attractive design.
Disadvantages
- In order to achieve good stability and control performance, several compromises need to be made, leading to a potential reduction in pure performance.
- The center of gravity allowable range is small, and must be precisely defined.
- Any control surface movement will affect the ideal lift distribution on the wing, producing secondary effects, like an increase in induced drag.
- Adverse yaw can be a big issue. However, there are new developments helping us, thanks to Albion Bowers! See Resources below.
- Pitch damping is an issue, due to the very small inertia on the lateral axis. PIO (pilot induced oscillations) are not rare for flying wing designs.
- Lateral stability can be an issue.
- It is true that there is only a wing to be built but, when it comes to swept wings, difficult aero/structural challenges come into the picture.
I’m convinced that some of these critical factors can be solved by a well thought design optimisation. In a tailless wing design, is very difficult to get the right trade-off between good performance, easy handling, and low production costs, when compared to traditional designs.
I’m very happy to see that this architecture is a bit revitalized nowadays (see Prandtl wing study at NASA, by Albion Bowers, Armstrong Flight Research Center Chief Scientist, in Resources) but the number of airplanes that will use this configuration will be still niche compared to the traditional architectures.
Nevertheless, the interest regarding some of the advantages given by that architecture, and the fashion connected to it, makes flying wing and tailless gliders far from disappearing from the scene.
I’ve always been attracted by both the design and the challenges connected to this configuration, and this is the reason why a started dreaming of my own tailless glider design.
The Design Evolution: First Concepts
Inspiration came from the Swift foot-launched glider (see Resources), that still remains the most successful tailless ultralight glider that reached the market. I wanted to see if it was possible to design a better streamlined sailplane, still keeping the advantage of the foot-launch method. In the meantime, Swift reached its third design evolution, raising again the bar!
Back in 2000, I started with a quite conservative aspect ratio and a thick wing section. One of the biggest issues was to find the right compromise for the pilot position with respect to the wing spar, and minimising the center of gravity shift between the pilot ‘running’ and the ‘seated’ configuration. The lift distribution over the wing was close to elliptical, and two big winglets were implemented.
In 2001–2002 I ran some aerodynamic and stability simulations. At that time, there was no ‘easy-to-use’ software for that. Searching on the web, I was able to find some freely available basic VLM code (vortex lattice method) in FORTRAN. Those codes were mainly coming from NASA and some US Universities. No GUI (graphic interface) was available at that time, and the software was quite complex. Nevertheless I was able to prove the concept.
In 2004 the design evolved to an higher aspect ratio wing, and the swept angle increased to 22°. The winglets were very nicely blended with the wing. The first free flight scale model was built and flown.
At that point, a great source of inspiration was Martin Hepperle’s website (see Resources below for link) and the wing section chosen for the wing was the MH-78. This wing section was specifically developed for foot-launched gliders.
In 2010–2011 I started investigating the prone pilot position: a fascinating configuration, but there are several drawbacks. The most annoying one is to find a good streamline for the fuselage.
After several studies, I decided to go for the traditional seated position, as shown on the right, above.
Shaping the Fuselage Pod
After a pause that lasted a couple of years, I focused on optimising the fuselage shape. I thought it was good to take as a reference some well known fuselage designs. The one below is belongs to the Rollanden Schneider LS6:
The LS6 is quite an old glider, but this fuselage design has been used on several sailplanes, from LS4 to LS8, providing very good performance. The difficulty with my design, is due to the fact that, more than a fuselage, i just need a pilot pod, since the glider will be a tailless one. So, I needed to optimize the shape in order to provide a good pressure gradient recovery on a reduced longitudinal length. The first conceptual design was still having some issues to be addressed (see picture immediately below). The wing incidence at the root was not optimised, the wing intersection with the pod was too much in the front, leading to a difficult blending of the wing, and the adverse pressure recovery at the end of the pod was critical.
After some more iterations, this is what the design was looking like:
You may recognize there is quite an angle, almost 7°, between fuse centreline and wing root chord. The wing is heavily twisted (we will see it later), and this brings 7° root chord incidence in trimmed conditions.
Up to this development stage, all decisions has been made considering a full scale aircraft: this is the reason why the fuselage pod is so big when compared to RC scale sailplanes. My final objective was, and it still is, to build a full-scale sailplane for myself. The financials connected with that are huge, and I do not know if I will be able to manage it at a certain point of time. But dreams are what drives our inspiration and commitment to work on personal projects, right?
In Part II of this series I will cover the design optimization of both wing and fuselage pod. Reynolds number plays an heavy role on the selection of wing profiles and fuselage shapes. What i will share in part II will be related to the scale model. Hoping you find this interesting, see you next time!
©2022 Domenico Bosco
Resources
- On Wings of the Minimum Induced Drag: Spanload Implications for Aircraft and Birds by Albion Bowers et al — “For nearly a century Ludwig Prandtl’s lifting-line theory remains a standard tool for understanding and analyzing aircraft wings. The tool, said Prandtl, initially points to the elliptical spanload as the most efficient wing choice…”
- Aerodynamics of Model Aircraft by Martin Hepperle — “This is a web site about model aircraft, airfoils, propellers and aerodynamics.…”
- Horten Flying Wings Believers on Facebook — This group is a good source of inspiration: “Place your Horten work here and tell about the positive yaw instead of adverse yaw. Tell about the lightness of the spar, tell about the great looks, tell about test you have done…”
- Tailless Aircraft in Theory and Practice by Karl Nickel and Michael Wohlfahrt — For the description of advantages and disadvantages, I took inspiration from the book: “discusses the full range of tailless designs, from hanggliders to the US ‘Stealth Bomber’, and includes a detailed look at particularly significant designs…”
- Aériane Swift From Wikipedia: — “The Aériane Swift is a lightweight (48 kg) foot-launched tailless sailplane whose rigid wings have a span of 40 feet. The Swift has been succeeded by the Swift’Lite. Although designed in California, Swift aircraft are now manufactured by Aériane, a European firm based in Gembloux, Belgium…”
All images by the author. Read the next article in this issue, return to the previous article in this issue or go to the table of contents. A PDF version of this article, or the entire issue, is available upon request.
